Thermal solution for a mezzanine card

ABSTRACT

A bottom heat dissipating device may be attached to a bottom surface of a printed circuit board (PCB). A top surface of the bottom heat dissipating device may be thermally coupled with a backside surface of one or more electronic components mounted on the bottom surface of the PCB. A top heat dissipating device may be attached to a top surface of the PCB. The top heat dissipating device may be thermally coupled with the bottom heat dissipating device through a thermally conductive coupling member to provide a conductive path for heat transfer from the bottom heat dissipating device to the top heat dissipating device. An opening adjacent to an edge of the thermally conductive coupling member may allow air flow between the top and bottom heat dissipating devices. The PCB may be part of a mezzanine card, such as a Peripheral Component Interconnect (PCI) mezzanine card (PMC).

FIELD OF THE INVENTION

The present invention relates to printed circuit board (PCB) packagetechnology and more particularly to a method and apparatus for removingheat from a PCB.

BACKGROUND OF THE INVENTION

Mezzanine cards allow for a variety of optional functional expansionsfor a host computer system, such as network interfacing, modemcommunications and security co-processing. Mezzanine cards may evencontain a main processor for the host system. Typically, a mezzaninecard is connected to a carrier board of the host system using thePeripheral Component Interconnect (PCI) bus for electricalinterconnections. Because mezzanine cards typically mount horizontallyto the carrier board, they may require less space than traditional PCIexpansion cards that mount vertically, at a right angle to amotherboard. Thus, mezzanine cards may be used in applications requiringrelatively small enclosures, such as set top boxes and internetappliances.

FIG. 1 illustrates an exemplary embodiment of a mezzanine systemaccording to the prior art, comprising a mezzanine card 100 mounted on acarrier board 104 by standoffs 106. The mezzanine card 100 may comprisea printed circuit board (PCB) 110 having electronic components 112mounted on a bottom surface of the mezzanine card 100. PCI busconnections may be made with connectors 114, which may mount withcorresponding connectors (not shown) on the carrier board. The carrierboard may be mounted in an enclosure 120 which may have a faceplate 122with an opening 124 to receive a bezel 116 of the mezzanine card 100. Aconnector 118 may be attached to the bezel 116 to provide an interfaceto the mezzanine card 100, such as an Ethernet network connection.

Typically, the majority of electronic components of a mezzanine card aremounted on the bottom surface of the PCB. Because the electroniccomponents may generate a substantial amount of heat, a heat dissipatingdevice, such as a bottom heat spreader 130, may be mounted on the bottomsurface of the PCB 110 to transfer heat from the electronic componentswhich may then be dissipated into the air. However, the surface area ofthe bottom heat spreader 130 may not be sufficient to adequately coolthe mezzanine card 100. Therefore, one of the challenges facingdesigners is to provide a thermal solution with an increased dissipativesurface area. However, physical dimensions of the mezzanine card 100 maybe defined by a standard that specifies a volumetric envelope throughwhich no components may protrude. Therefore, another challenge facingdesigners is to ensure the thermal solution fits within a standardvolumetric envelope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary mezzanine system and mezzanine cardaccording to the prior art.

FIG. 2 illustrates a flow diagram of an exemplary method according toone embodiment of the present invention.

FIGS. 3A and 3B illustrate side views of an exemplary embodiment of amezzanine card including a coupling member, according to one embodimentof the present invention;

FIG. 4 illustrates an exploded view of an exemplary embodiment of amezzanine card according to one embodiment of the present invention.

FIG. 5 illustrates a top view of an exemplary embodiment of a heatspreader according to one embodiment of the present invention.

FIG. 6 illustrates a top view of an exemplary embodiment of a mezzaninecard according to one embodiment of the present invention.

FIG. 7 illustrates a flow diagram of an exemplary method to optimizecoupling member dimensions according to one embodiment of the presentinvention.

FIG. 8 illustrates a side view of an exemplary embodiment of a couplingmember according to one embodiment of the present invention.

FIG. 9 illustrates an exemplary volumetric envelope of a mezzanine cardassembly according to one embodiment of the present invention.

DETAILED DESCRIPTION

The following detailed description sets forth an embodiment orembodiments in accordance with the present invention. In the followingdescription, numerous specific details are set forth such as materialtypes, dimensions, and ranges, in order to provide a thoroughunderstanding of the present invention. However, it will be obvious toone of skill in the art, that the invention may be practiced withoutthese specific details. In other instances, well-known elements andprocessing techniques have not been shown in particular detail in orderto avoid unnecessarily obscuring the present invention.

Some embodiments of the present invention provide a method and apparatusto remove heat from a printed circuit board (PCB) by a combination ofconduction and convection. A bottom heat dissipating device may bethermally coupled with one or more electronic devices mounted on abottom surface of the PCB. A top heat dissipating device may bethermally coupled with the bottom heat dissipating device through athermally conductive coupling member to provide a conductive path forheat transfer from the bottom heat dissipating device to the top heatdissipating device. The additional dissipative surface area of the topheat dissipating device may assist in removing heat from the PCB.Openings adjacent to the coupling member may allow air flow between thetop and bottom heat dissipating devices, which may further assist inremoving heat from the PCB by convection.

FIG. 2 is a flow diagram 200 illustrating exemplary operations of amethod to remove heat from a PCB according to one embodiment of thepresent invention. The operations of FIG. 2 may be described withreference to the exemplary embodiments of FIGS. 3 and 4, whichillustrate an exemplary mezzanine card 300 and an exploded view of theexemplary mezzanine card 300, respectively. However, it should beunderstood that performing the exemplary operations of FIG. 2 may resultin embodiments other than the exemplary embodiments of FIGS. 3 and 4.For example, the operations of FIG. 2 may be applied to other mezzaninecards, and generally, to other types of PCB assemblies.

For block 210, a bottom heat dissipating device is attached to a bottomsurface of a PCB, a top surface of the bottom heat dissipating devicethermally coupled with a backside surface of one or more electroniccomponents mounted on the bottom surface of the PCB. The bottom heatdissipating device may be any suitable heat dissipating device, such asa heat sink, thermal plate, or heat spreader.

For example, with reference to the exemplary mezzanine card 300 of FIG.3, a bottom heat spreader 330 may be attached to a bottom surface of aPCB 310, with a top surface of the bottom heat spreader 330 thermallycoupled with a backside surface of one or more electronic components 312mounted on the bottom surface of the PCB 310. The mezzanine card 300 maybe mounted on a carrier board 304 with mounting hardware, such asstandoffs 306 and a bezel 316, with the bottom heat spreader 330 facingthe carrier board 304. Electrical interconnections between the mezzaninecard 300 and the carrier board 304 may be made through connectors 314,which may provide PCI bus interconnections to the mezzanine card 300.

The electronic components 312 may comprise any suitable electroniccomponents to perform a desired function of the mezzanine card 300, suchas network interfacing or digital signal processing (DSP). For oneembodiment of the present invention, the electronic components 312 maycomprise a processor, a chipset such as a memory controller chipset,and/or a memory.

For one embodiment of the present invention, a thermal interfacematerial (TIM) may be disposed between a backside surface of the one ormore electronic components 312 and the top surface of the bottom heatspreader 330 in an effort to enhance heat transfer. For example, athermal pad 432, illustrated in the exploded view of FIG. 4, may bedisposed between the top surface of the bottom heat spreader 330 and thebackside surface of the electronic components 312. Other types ofsuitable TIMs may also be used, such as thermal greases, thermaladhesives, and thermal gels.

The bottom heat spreader 330 may be made of any suitable thermallyconductive material, such as a metal or a thermally conductive compositematerial. For example, the bottom heat spreader 330 may be made ofcopper, a copper alloy, aluminum, or an aluminum alloy. The bottom heatspreader may be formed from a relatively inexpensive sheet metal. Thebottom heat spreader may be formed with surface area increasingfeatures, such as fins. As illustrated in FIG. 5, for one embodiment ofthe present invention, the bottom heat spreader may have recessed areas502 and 504 having different depths to thermally couple with electroniccomponents having different heights.

For block 220 of FIG. 2, a top heat dissipating device is attached to atop surface of the PCB. For example, a top heat spreader 340 may beattached to a top surface of the PCB 310. The top heat spreader 340 mayalso be made of any suitable thermally conductive material. For oneembodiment of the present invention, the top heat spreader 340 may bemade of the same material as the bottom heat spreader 330. For oneembodiment of the present invention, the top and bottom heat spreadersmay have substantially identical physical dimensions, allowing them tobe interchanged, which may reduce the assembly time of the mezzaninecard.

For one embodiment of the present invention, the top heat spreader 340may be thermally coupled with one or more electronic components 318mounted on the top surface of the PCB 310. For example, the electroniccomponents 318 may comprise optional memory devices mounted on the topsurface of the PCB 310. A TIM may be disposed between a backside surfaceof the electronic components 318 and a bottom surface of the top heatspreader 340, as previously described with reference to the bottom heatspreader 330.

For block 230 of FIG. 2, the top heat dissipating device is thermallycoupled with the bottom heat dissipating device through a thermallyconductive coupling member. The thermally conductive coupling member maybe a variety of shapes and configurations, and may be located at variousplaces along the PCB.

For example, the top heat spreader 340 may be thermally coupled with thebottom heat spreader 330 through a thermally conductive coupling member350, which may be located along an edge of the PCB 310. The couplingmember 350 maybe made of any suitable thermally conductive material. Forexample, the coupling member 350 may be made of the same material as thebottom heat spreader 330 and/or the top heat spreader 340. Asillustrated in the exploded view of FIG. 4, the coupling member 350 maycomprise a top portion 352 and a bottom portion 354 which may beintegrated with the top heat spreader 340 and the bottom heat spreader330, respectively. The top portion 352 may mate with the bottom portion354, for example, when the top and bottom heat spreaders are attached tothe PCB 310.

The bottom heat spreader 330 and top heat spreader 340 may be attachedto the PCB 310 according to any suitable attachment methods, forexample, using general adhesives, thermal adhesives, and/or mechanicalclips. For one embodiment of the present invention, as illustrated inFIGS. 3 and 4, the bottom heat spreader 330 and top heat spreader 340may be attached to the PCB 310 with mounting hardware used to mount themezzanine card 300 to the carrier board 304.

For example, the top and bottom heat spreaders may have mounting tabs444 and 434, respectively, with holes (not shown) through which screws406 and 416 may be inserted to couple with internal threads of thestandoffs 306 and the bezel 316, respectively. An advantage to mountingthe heat spreaders with the mezzanine card mounting hardware is that theheat spreaders are easily removed, for example, in the case of anupgrade or repair.

For some embodiments of the present invention, the mounting hardware(screws, standoffs, etc.) may be made of a thermally conductivematerial. Therefore, another advantage to mounting the heat spreaderswith the mezzanine card mounting hardware is that thermally conductivemounting hardware may provide additional conductive paths for heattransfer from the bottom heat spreader to the top heat spreader. Athermal interface material (TIM), such as a thermal gel, may be appliedto mounting hardware to enhance heat transfer through the mountinghardware. Alternatively, the heat spreaders may be attached to the PCBwith rivets or solder, which may also be thermally conductive.

For some embodiments of the present invention, an electronic componentthat generates a substantial amount of heat, such as a processor, may beplaced near a coupling member in an effort to minimize a path for heattransfer from the electronic component to a top heat spreader. Forexample, FIG. 6 illustrates a bottom view of a PCB 610 having anelectronic component 612 placed within 3 millimeter (mm) of an edgeportion 602 of the PCB 610 near a coupling member 650, thermally coupledwith a top heat spreader 640. A bottom heat spreader is not shown inFIG. 6 to allow illustration of a bottom surface of the PCB 610.

CONVENTION COOLING

Some embodiments of the present invention may provide improved coolingof a PCB through a combination of conduction and convection cooling. Asillustrated in FIG. 3, openings 360 may be formed adjacent to side edges356 of the coupling member 350. The openings 360 may allow air flowbetween the top and bottom heat spreaders, which may help cool the PCB310 of the mezzanine card 300 by convection.

Convection may be governed by the following equation for volume flowrate (Vrate) of air through the area of the openings 360:

Vrate=v*h*(X 1-X 2)

where “v” is the velocity of the air, “h” and “X2” are a height andlength of the coupling member 350, respectively, and “X1” is a length ofthe top and bottom heat spreaders. For one embodiment of the presentinvention, the length X1 of the top and bottom heat spreaders may bebetween 100 and 140 mm, while the length X2 of the coupling member 350may be between 25 and 50 mm.

Conduction through the coupling member 350 may be governed by thefollowing equation for conductive transfer of heat (Q) dissipated by thePCB:

Q=K*t*X 2*(T 1-T 2)

where, K is a thermal conductivity of the coupling member 350, t is athickness of the coupling member 350, T1 and T2 are temperatures of thebottom heat spreader 330 and top heat spreader 340, respectively. Q maybe determined by the power dissipation of the PCB, which may be in therange of 5 to 20 W.

It may be seen from the governing equations for convection andconduction that while decreasing the length X2 of the coupling member350 may increase convection, it may also decrease conduction. Forexample, with no coupling member (X2=0), there may be maximumconvection, but no conductive path from the bottom heat spreader 330 tothe top heat spreader 340. Alternatively, if the length X2 of thecoupling member is equal to the length X1 of the top and bottom heatspreaders, there may be no openings 360 and, hence, no air flow(Vrate=0).

OPTIMIZING DIMENSIONS OF THE COUPLING MEMBER

Because a mezzanine system may be packaged in a small enclosure, withoutroom for a cooling fan, the velocity of air may be limited to free airconvection, which may be in the range of 0.1 to 0.15 m/s. Optimizing thedimensions of the coupling member may allow mezzanine cards with highpower processors to be sufficiently cooled by the small amount of airflow generated by free air convection.

For example, it may be seen from the governing equation for conductionlisted above, that if Q, K and t are held constant, different values ofX2 may produce different values for the temperature T1 of the bottomheat spreader and the temperature T2 of the top heat spreader.Therefore, for some embodiments of the present invention, X2 may beoptimized in an effort to maintain the temperature of the bottom heatspreader T1 below a target temperature.

For example, FIG. 7 illustrates a flow diagram 700 of a method ofoptimizing a length (X2) of a coupling member according to oneembodiment of the present invention. FIG. 7 may be described withreference to the exemplary mezzanine card 300 illustrated in FIG. 3. Theoperations of flow diagram 700 may be easily modified to optimize otherdimensions, such as the thickness (t) of the coupling member, the height(h) of the coupling member and/or a length (X1) of top and bottom heatspreaders.

For block 710, constant values are determined for heat generated byelectronic components, a length of top and bottom heat dissipatingdevices, and a thickness and height of a coupling member. For example,the heat generated by the electronic components 312 mounted on thebottom surface of the PCB 310 of the mezzanine card 300 may bedetermined by power dissipation of the mezzanine card, which may be inthe range of 5 to 15 W, with over 5 W generated by a processor alone.The length X1 of the top and bottom heat spreaders 340 and 330 may bedetermined by the location of the mounting hardware.

For block 720, a temperature of the bottom heat dissipating device ismeasured for various lengths of the coupling member between a minimumand a maximum value. For example, the actual temperature of the bottomheat spreader 330 may be measured by experimentation while operating themezzanine card 300. Alternatively, the temperature of the bottom heatspreader 330 may be measured by simulation. For example, the mezzaninecard 300 may be modeled using any suitable simulation tools, such asICEPAK electronics cooling modeling software available from Fluent Inc.,of Lebanon, N.H. Such simulation tools may produce accurate results byperforming simulations with accurate component packaging and wattages,as well as material properties for the PCB, heat spreaders and couplingmember.

For block 730, a length of the coupling member corresponding to ameasured temperature of the bottom heat dissipating device below atarget temperature is chosen. As previously described, the targettemperature may correspond to a maximum operating temperature of theelectronic components 312. For example, with approximate constant valuesof Q=10W, h=5.6 mm, t=2 mm, and X1=127 mm, an optimum value for X2 maybe approximately 38 mm to keep the temperature of the bottom heatspreader 330 below a target temperature of 95° C.

For some embodiments of the present invention, openings may be formed inthe coupling member itself in an effort to enhance convection. Forexample, FIG. 8 illustrates a side view 800 of a coupling member 850with openings 852 formed therein, which may allow air flow through abottom heat spreader 830 and a top heat spreader 840. Dimensions of thecoupling member 850 and the openings 852 may be optimized for conductionand convection cooling, as previously described.

For some embodiments of the present invention, multiple thermallyconductive coupling members may be used to thermally couple top andbottom heat dissipating devices. For example, multiple coupling membersmay be placed adjacent to each other, or along opposing edges of a PCB.For some embodiments of the present invention, a PCB may have anopening, or cut-out, to accommodate placement of a coupling memberperpendicular to the PCB, or even in the middle of the PCB. A cut-out inthe PCB, however, may reduce a signal routing area of the PCB, which mayincrease complexity of signal routing. Further, various mezzanine cardstandards may prohibit cut-out areas along the PCB.

COMMON MEZZANINE CARD (CMC) STANDARD ENVELOPE

For example, the Institute of Electrical and Electronics Engineers(IEEE) Standard for a Common Mezzanine Card Family: CMC, IEEE STD P1386(the IEEE 1386 standard), published on Jun. 14, 2001, defines physicaldimensions for common mezzanine cards, such as PCI mezzanine cards(PMCs). Compliance with a mezzanine card standard, such as the IEEE 1386standard, may enhance sales of a mezzanine card by ensuring itscompatibility with mezzanine systems manufacturers by various vendors.

The IEEE 1386 standard specifies a volumetric envelope which defines aspace that may be occupied by a mezzanine card including the PCB,associated electronic components, and a minimum cooling gap. The totalmezzanine card envelope may be divided into two parts, the input/output(I/O) envelope and the component envelope. The I/O envelope defineswhere I/O connectors may be mounted on the mezzanine card bezel. Thecomponent envelope defines where electronic components and the printedcircuit board may be placed. Because some mezzanine cards may not haveI/O connectors, components may be placed in the I/O envelope.

FIG. 9 illustrates a mezzanine card 900 in accordance with the IEEE 1386standard, having a standard width of 74 mm, and a standard depth of 149mm. The I/O envelope has a standard height of 13.5 mm and a standarddepth of 31.0 mm, while the component envelope has a standard height of8.2 mm. As illustrated, for one embodiment of the present invention, aPCB 910, a bottom heat spreader 930, a top heat spreader 940, and acoupling member 950 may fit within the standard component envelopeheight of 8.2 mm. As previously described, the coupling member 950 mayhave a length of approximately 38 mm (with a width of 2 mm) to provide abalance between conduction and convection. Other embodiments of thepresent invention may be in accordance with standards other than theIEEE 1386 standard, which may define different volumetric envelopes.Therefore, the above described dimensions may be modified accordingly.

In the foregoing description, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit or scope of the presentinvention as defined in the appended claims. For example, while theforegoing detailed description referred to mezzanine cards in detail, itshould be understood that the present invention may be applied to othertypes of PCB assemblies, such as memory cards, network cards, othertypes of expansion cards, etc. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense.

What is claimed is:
 1. An apparatus comprising: a printed circuit board(PCB) having a top surface and a bottom surface; at least one electroniccomponent, having a backside surface, mounted on the bottom surface ofthe PCB; a bottom heat dissipating device, having a bottom couplingportion, attached to the bottom surface of the PCB, a top surfacethereof thermally coupled to the backside surface of the electroniccomponent; and a top heat dissipating device, having a top couplingportion, attached to the top surface of the PCB, the top couplingportion contacting the bottom coupling portion, the bottom and topcoupling portions jointly forming a thermally conductive coupling memberbeing thermally coupled to the bottom and top heat dissipating devicesat least one of the heat dissipating devices having recessed areas ofdifferent depths to thermally couple with electronic components ofdifferent heights.
 2. The apparatus of claim 1, wherein the apparatus isa mezzanine card.
 3. The apparatus of claim 2, wherein the top andbottom heat dissipating devices are heat spreaders with lengths between100 and 140 millimeters.
 4. The apparatus of claim 3, wherein a heightfrom a bottom surface of the bottom heat spreader to a top surface ofthe top heat spreader is 8.2 millimeters or less.
 5. The apparatus ofclaim 3, wherein the one or more electronic components mounted on thebottom surface of the PCB comprise a processor.
 6. The apparatus ofclaim 5, wherein the thermally conductive coupling member extends alongan edge portion of the PCB, and the processor is mounted within 3millimeters from said edge portion.
 7. The apparatus of claim 1, whereinthe thermally conductive coupling member comprises a top portion and abottom portion, wherein the top portion is integral with the top heatdissipating device and/or the bottom portion is integral with the bottomheat dissipating device.
 8. The apparatus of claim 1, further comprisingat least one other thermally conductive coupling member thermallycoupled with the bottom and top heat dissipating devices.
 9. Theapparatus of claim 1, wherein the bottom heat dissipating device and/orthe top heat dissipating device are made substantially of copper or acopper alloy.
 10. The apparatus of claim 1, wherein a bottom surface ofthe top heat dissipating device is thermally coupled with one or moreelectronic devices mounted on the top surface of the PCB.
 11. Theapparatus of claim 1, wherein the top and bottom heat dissipatingdevices are attached to the PCB with thermally conductive mountinghardware, said mounting hardware thermally coupled with the top andbottom heat dissipating devices.
 12. The apparatus of claim 11, whereinthe thermally conductive mounting hardware comprises one or more screws.13. The apparatus of claim 1, wherein the thermally conductive couplingmember extends partially adjacent a side edge of the PCB between the topand bottom heat dissipating devices.
 14. The apparatus of claim 13,wherein lengths of the top and bottom heat dissipating devices extendingalong the edge of the PCB are between 100 and 140 millimeters and alength of the thermally conductive coupling member extending along theedge is between 25 and 50 millimeters.
 15. The apparatus of claim 1,wherein the thermally conductive coupling member extends along an edgeof the PCB with at least one opening formed within the thermallyconductive coupling member.
 16. A system comprising: a carrier boardhaving a bus; and a mezzanine card mounted on the carrier board coupledwith the bus, the mezzanine card comprising a printed circuit board(PCB), a bottom heat dissipating device attached to a bottom surface ofthe PCB facing the carrier board, a top surface of the bottom heatdissipating device thermally coupled with a backside surface of one ormore electronic components mounted on the bottom surface of the PCB, atop heat dissipating device attached to a top surface of the PCB, and athermally conductive coupling member thermally coupled with the bottomand top heat dissipating device, the thermally conductive couplingmember having a portion integral with at least one of the heatdissipating devices and being adjacent to the other heat dissipatingdevice, the thermally conductive coupling member having a length beingbetween 17 and 50 percent of a length of one of the heat dissipatingdevices.
 17. The system of claim 16, wherein the top and bottom heatdissipating devices are heat spreaders having a length between 100 and140 millimeters.
 18. The system of claim 16, wherein a height from a topsurface of the carrier board to a top surface of the top heatdissipating device is 13.5 millimeters or less.
 19. The system of claim16, wherein the bottom heat dissipating device and/or the top heatdissipating device have recessed areas of different depths to thermallycouple with electronic components of different heights.
 20. The systemof claim 16, wherein the top and bottom heat dissipating devices areattached to the PCB with thermally conductive mounting hardware alsoused to mount the mezzanine card to the carrier board, said mountinghardware thermally coupled with the top and bottom heat dissipatingdevices.
 21. The system of claim 16, wherein the thermally conductivecoupling member extends partially adjacent to a side edge of the PCBbetween the top and bottom heat dissipating devices.
 22. The system ofclaim 21, wherein lengths of the top and bottom heat dissipating devicesextending along the edge of the PCB are between 100 and 140 millimetersand a length of the thermally conductive coupling member extending alongthe edge is between 25 and 50 millimeters.